In recent years composite materials have become increasingly popular for use in a variety of aerospace applications because of their durability and relative lightweight. Although composite materials can provide superior strength and weight properties, designing flanges on structures fabricated from composite materials still remains a challenge.
Current flange lay-up processes can generally involve repeatedly applying plies, or layers, of fabric to a composite structure until a flange having the desired dimensions is obtained. One issue that can arise during this process is that when a second ply of fabric is applied, it can cover the first ply of fabric, thereby making it nearly impossible to monitor the first ply to ensure its placement is unchanged. As a result, bridging of the fabric may occur.
Bridging is when the initial, or first, fabric ply pulls away from the flange corner and spans across the corner rather than remaining tightly adhered thereto. Bridging may result in resin richness, which is an undesired agglomeration of excess resin beneath the first ply of fabric that can locally weaken the laminate.
Bridging may be caused by any of several factors. For example, bridging may result from inadequately placing the initial fabric plies into the corner of the flange such that as subsequent plies are applied during lay-up the initial plies may be jostled causing bridging. Also, inadequately removing bulk from the fabric plies during layup can result in an excess length of composite material, which can lead to bridging during the curing process. Additionally, bridging may result from differences in thermal expansion of the tooling versus the fabric during the curing process.
Regardless of the cause, there is currently no way for operators to easily monitor the initial fabric ply once it has been covered to help ensure that bridging is prevented and that the first ply of fabric remains adhered in the flange corner. The best current practice is to terminate the fabric plies at the corner to allow fabric slippage. However, this practice is generally only beneficial in addressing the issue of differences in thermal expansion during cure and does nothing to prevent bridging that occurs as a result of jostling or inadequate bulk removal.
Accordingly, it would be desirable to produce a composite flange that is less susceptible to bridging and the previously described fabrication concerns.